Strength of Dry and Wet Quartz in the Low-Temperature Plasticity Regime: Insights From Nanoindentation.

At low-temperature and high-stress conditions, quartz deformation is controlled by the kinetics of dislocation glide, that is, low-temperature plasticity (LTP). To investigate the relationship between intracrystalline H2O content and the yield strength of quartz LTP, we have integrated spherical and Berkovich nanoindentation tests at room temperature on natural quartz with electron backscatter diffraction and secondary-ion mass spectrometry measurements of intracrystalline H2O content. Dry (<20 wt ppm H2O) and wet (20-100 wt ppm H2O) crystals exhibit comparable indentation hardness. Quartz yield strength, which is proportional to indentation hardness, seems to be unaffected by the intracrystalline H2O content when deformed under room temperature, high-stress conditions. Pre-indentation intracrystalline microstructure may have provided a high density of dislocation sources, influencing the first increments of low-temperature plastic strains. Our results have implications for fault strength at the frictional-viscous transition and during transient deformation by LTP, such as seismogenic loading and post-seismic creep.

The earthquake cycle in the dry lower continental crust: insights from two deeply exhumed terranes (Musgrave Ranges, Australia and Lofoten, Norway).

This paper discusses the results of field-based geological investigations of exhumed rocks exposed in the Musgrave Ranges (Central Australia) and in Nusfjord (Lofoten, Norway) that preserve evidence for lower continental crustal earthquakes with focal depths of approximately 25-40 km. These studies have established that deformation of the dry lower continental crust is characterized by a cyclic interplay between viscous creep (mylonitization) and brittle, seismic slip associated with the formation of pseudotachylytes (a solidified melt produced during seismic slip along a fault in silicate rocks). Seismic slip triggers rheological weakening and a transition to viscous creep, which may be already active during the immediate post-seismic deformation along faults initially characterized by frictional melting and wall-rock damage. The cyclical interplay between seismic slip and viscous creep implies transient oscillations in stress and strain rate, which are preserved in the shear zone microstructure. In both localities, the spatial distribution of pseudotachylytes is consistent with a local (deep) source for the transient high stresses required to generate earthquakes in the lower crust. This deep source is the result of localized stress amplification in dry and strong materials generated at the contacts with ductile shear zones, producing multiple generations of pseudotachylyte over geological time. This implies that both the short- and the long-term rheological evolution of the dry lower crust typical of continental interiors is controlled by earthquake cycle deformation. This article is part of a discussion meeting issue 'Understanding earthquakes using the geological record'.

On-fault earthquake energy density partitioning from shocked garnet in an exhumed seismic midcrustal fault.

The energy released during an earthquake is mostly dissipated in the fault zone and subordinately as radiated seismic waves. The on-fault energy budget is partitioned into frictional heat, generation of new grain surface by microfracturing, and crystal-lattice distortion associated with dislocation defects. The relative contribution of these components is debated and difficult to assess, but this energy partitioning strongly influences earthquake mechanics. We use high-resolution scanning-electron-microscopy techniques, especially to analyze shocked garnet in a fault wall-rock, to provide the first estimate of all three energy components for a seismic fault patch exhumed from midcrustal conditions. Fault single-jerk seismicity is recorded by the presence of pristine quenched frictional melt. The estimated value of energy per unit fault surface is ~13 megajoules per square meter for heat, which is predominant with respect to both surface energy (up to 0.29 megajoules per square meter) and energy associated with crystal lattice distortion (0.02 megajoules per square meter).

Methane-hydrogen-rich fluid migration may trigger seismic failure in subduction zones at forearc depths.

Metamorphic fluids, faults, and shear zones are carriers of carbon from the deep Earth to shallower reservoirs. Some of these fluids are reduced and transport energy sources, like H2 and light hydrocarbons. Mechanisms and pathways capable of transporting these deep energy sources towards shallower reservoirs remain unidentified. Here we present geological evidence of failure of mechanically strong rocks due to the accumulation of CH4-H2-rich fluids at deep forearc depths, which ultimately reached supralithostatic pore fluid pressure. These fluids originated from adjacent reduction of carbonates by H2-rich fluids during serpentinization at eclogite-to-blueschist-facies conditions. Thermodynamic modeling predicts that the production and accumulation of CH4-H2-rich aqueous fluids can produce fluid overpressure more easily than carbon-poor and CO2-rich aqueous fluids. This study provides evidence for the migration of deep Earth energy sources along tectonic discontinuities, and suggests causal relationships with brittle failure of hard rock types that may trigger seismic activity at forearc depths.

Feedback between high-pressure genesis of abiotic methane and strain localization in subducted carbonate rocks.

Fluid-rock interactions exert key control over rock rheology and strain localization. Redox may significantly affect the reaction pathways and, thereby, the mechanical properties of the rock. This effect may become critical in volatile-rich, redox sensitive rocks such as carbonate-rich lithologies, the breakdown of which can significantly modify the net volume change of fluid-mediated reactions. Subduction focus the largest recycling of crustal carbonates and the most intense seismic activity on Earth. Nevertheless, the feedbacks between deep carbon mobilization and deformation remain poorly investigated. We present quantitative microstructural results from natural samples and thermodynamic modeling indicating that percolation of reducing fluids exerts strong control on the mobilization of carbon and on strain localization in subducted carbonate rocks. Fluid-mediated carbonate reduction progressed from discrete domains unaffected by ductile deformation into localized shear zones deforming via diffusion creep, dissolution-precipitation creep and grain boundary sliding. Grain-size reduction and creep cavitation along localized shear zones enhanced fluid-carbonate interactions and fluid channelization. These results indicate that reduction of carbonate rocks can exert an important positive feedback on strain localization and fluid channelization, with potential implications on seismic activity and transport of deep hydrocarbon-bearing fluids.